One of the major uses of satellite communication systems that are now in operation is the completion of telephone circuits between geographically remote locations.
These conventional terrestrial telephone circuits usually carry supervisory information indicating the status of the circuit. This supervisory signal often is in the form of a DC voltage level or a single frequency tone. When the circuit is idle, a level or frequency tone is placed on the line. Signaling is then accomplished by interrupting the DC path or pulsing the tone frequency, respectively. Signaling is employed to indicate the status of the telephone receiver by, for instance, detecting the condition of the hook switch. Thus, if the receiver is "on hook" a DC level is placed on the line to indicate an idle line. When the receiver goes "off hook" the line may be grounded. Similarly, dial pulsing is reflected by pulsing the condition of the line, with one pulse transmitted for each respective dial position. Thus, for instance, when one dials the number 7, 7 pulses may be transmitted on the line representative of that dialing action.
Known satellite communication systems which are today employed for completing telephone circuits separate the telephone signaling and supervisory information from the information which the subscriber transmits by speaking into the telephone hand set. Thus, for instance, at any terminal transmitting to a satellite, a channel is completed for each different active subscriber, for the information being transmitted by the subscriber, and a common channel is employed for supervisory and signaling information. In those communication systems which employ demand assignment features, the signaling channel is required, if only for the purposes of effecting demand assignment. There are, however, systems which do not perform demand assignment, i.e., dedicated channels. These channels obviously do not require a signaling channel for demand assignment functions, and therefore this channel carries only supervisory and other signaling information. Based on the amount and type of traffic carried by a particular transmitting terminal, the use of this channel for supervisory and signaling information may or may not be an efficient use of channel capacity. Thus, there has been a desire for satellite communication systems which employs so-called "in-band" supervisory and signaling functions. Such systems are characterized by assigning a channel to each active subscriber for not only transmitting the subscriber originated information, such as the audio portion of a telephone message, but for transmitting supervisory and signaling information as well.
It should be apparent to those skilled in the art that in any satellite communication system, one limiting parameter on the capacity of the system is the transmitting power available at the satellite. It should also be apparent that for single channel per carrier systems, it is wasteful of this limited satellite power to transmit line status in a continuous manner. To eliminate this requirement, and still provide the inband supervisory and signaling information transmitting capability, interfacing equipment is required to detect the condition of the telephone line and format a non-continuous message containing information as to the status of the telephone line for transmission. At the receiving terminal, a complimentary interface is required to detect the information contained in the supervisory and signaling messages formatted by the transmitting interface and to convert that information to a form which is capable of being employed by the receiving conventional terrestrial telephone system.
Satellite communication systems, in linking geographically widely separated telephone systems, perform a highly desirable function. However, this very function may well introduce additional complexities. Thus, for instance, the linking of a telephone system in one country with the telephone system in a second country can introduce problems when the terrestrial signaling systems in the two countries are not compatible. For instance, if the transmitting country maintains a highly sophisticated and relatively fast system in which dial pulse information is sent at a rapid rate, a problem may be introduced if the telephone system in the receiving country cannot accept dial pulse information at the rapid transmitting rate. To remedy this compatibility problems, some means must be provided to "slow down" the rapid transmission of information so that it can be effectively received by the receiving telephone system.
Moreover, since there are many different telephone systems to be interconnected by satellite communication systems, this may call for a different dial pulse rate translation, for example, for each different pair of telephone systems. Obviously, it is inefficient to design and build a different dial pulse rate translation equipment for each possible different pair of telephone systems which are to be interconnected. Rather, it would be highly desirable to design, develop and manufacture a single such dial pulse rate translation equipment which could simply be adjusted for providing the necessary translation between almost any different pair of telephone systems which are capable of being interconnected.
It is therefore one object of the present invention to provide interfacing equipment to accept supervisory and signaling information from a telephone system, format that received information into messages for effective satellite transmission, receive the transmitted messages and re-format the information so transmitted into a form for effective reception by a different telephone system. It is another object of the present invention to provide the foregoing functions which eliminate the necessity for transmitting DC levels or continuous tones which transmissions are either wasteful of limited satellite power trnsmitting capacity or are not physically realizable. It is another object of the invention to provide an interface for transmitting information on line availability in a non-continuous fashion.
It is another object of the present invention to provide such interfacing equipment which, at the same time, is capable of translating the rate at which signaling information is communicated. It is still another object of the present invention to provide the aforementioned interfacing equipment which is capable of providing differing signaling pulse rate translations as determined by the particular application.
These and other objects of the invention are fulfilled by the interfacing equipment of this invention.